The meters are most frequently used to measure the water content in soil or rock.
The technique is non-destructive, and is sensitive to moisture in the bulk of the target material, not just at the surface.
With a source of high-energy neutrons and a detector sensitive to low-energy neutrons (thermal neutrons), the detection rate will be governed by the water content of the soil between the source and the detector.
The neutron source typically contains a small amount of a radionuclide.
Sources may emit neutrons during spontaneous fission, as with californium; alternatively, an alpha emitter may be mixed with a light element for a nuclear reaction yielding excess neutrons, as with americium in a beryllium matrix.